In GB 2191606A and U.S. Pat. No. 5,005,353 there is described a method of continuous active control in turbomachinery of unsteady motion phenomena such as blade flutter, surge and rotating stall in turbocompressors. The control signals generated by the method described in that earlier patent application are applied in any of a number of possible ways to achieve the desired end. Specifically, the signals may activate actuators which are disposed in the compressor itself by being mounted in the casing wall enclosing the rotary spool with its stators. A variety of devices are proposed for such actuators, including loudspeakers for producing pressure waves, flow injection and bleed valves, heat injection means, and drives for variable angle aerodynamic elements such as stator vanes. By these means symmetric or non-symmetric distortions of the gasflow are generated to ameliorate disturbances sensed in the flow through the compressor.
These different devices are capable of dealing with various forms of flow instability. However, their incorporation in turbomachinery can bring design problems because of the space that must be made available for them. This can be a particular problem in the turbocompressors of aero-engines where accommodating the additional equipment may be deleterious to the aerodynamic efficiency of the engine.